This irregular shaped bump can be a cyst, swollen lymph node or a benign skin growth. A sebaceous cyst is a closed sac occurring just under the skin which contains a "pasty" or "cheesy" looking substance called keratin.They may occur anywhere on body but scalp, ears, back, face, and upper arm, are common sites for sebaceous cysts. I would suggest you to get a biopsy skin and a fine needle aspiration cytology done to confirm the diagnosis. Treatment is diagnosis specific.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
i had an x ray on my arm after dropping a heavy weight on it the doctor at the time said it was fine and no breaks but now i have aletter through the post saying that a specialist has taken a loook and would like me to come in for a review and im very worried should i be